The development of intravascular monitoring probes has created a need for a device that can be quickly and easily attached to a preplaced small gauge catheter and then operated to insert the probe through the catheter and into a blood vessel. Present methods and devices for inserting a catheter within a blood vessel are unsuitable for the delivery of probes for a number of reasons. First, these existing devices have no means to prevent the leakage of bodily fluids from the catheter as the probe is inserted into and withdrawn from the blood vessel. In addition, these prior art devices provide no secondary fluid path for pressure monitoring, blood sampling, or for the introduction of additional fluids into the artery. Furthermore, they do not provide means to center the probe within the catheter lumen while advancing the probe into the Blood vessel.